This invention relates to electric arc welding and, more particularly, to apparatus for providing scheduled variable polarity direct electric current between an electrode and a workpiece surface.
A variety of direct current (D.C.) electric arc welding methods and apparatus have been reported. As they relate to a form of the present invention, those describing D.C. plasma transferred arc methods and apparatus are of interest. In a typical arrangement, a form of surfacing using D.C. plasma arc welding has been described in U.S. Pat. No. 4,125,754-Wasserman (patented Nov. 14, 1978). In that patent, a plasma arc is generated between a non-consumable electrode always as negative and a workpiece always as positive, while powder is introduced into the plasma arc and carried toward the workpiece. In U.S. Pat. No. 4,689,463-Shubert (patented Aug. 25, 1987), that same general arrangement maintains the electrode always as negative and the workpiece always as positive while pulsing, periodically, the current between a main current level and a lower, auxiliary current level.
Still another arrangement, related to and a modification of the above Shubert patent, is shown in U.S. Pat. No. 4,739,146- Lindland and Shubert (patented Apr. 19, 1988). In the '146 patent, the pulsing current of the '463 patent is pulsed or switched at a high frequency, (compare FIG. 1 of each of those patents), for example about 15,000 cycles per second. Allegedly this is done to stabilize the current and to obtain a more effective power output for a given amperage input.
All of the above identified, and similar, D.C. arrangements are known in the art as straight polarity D.C. methods and apparatus because, throughout operation, the polarity of the electrode remains the same polarity, as does the polarity of the workpiece surface. The disclosures of each of the above identified patents are hereby incorporated herein by reference for background purposes and to show typical reported arrangements.